The present invention relates to a temperature sensor circuit that includes a depletion mode MOS.
FIG. 7 is a circuit diagram showing a conventional temperature sensor circuit. In a conventional temperature sensor circuit 400, the collector of a first bipolar transistor 3 is connected to a power supply 1. The emitter of the first bipolar transistor 3 is connected to the drain of depletion mode NMOS 4, the gate and source thereof are short-circuited to each other. The source of depletion mode NMOS 4 is connected to ground 2. The base of the first bipolar transistor 3 is connected to the emitter of the second bipolar transistor 5, the collector thereof is connected to power supply 1. The base of the second bipolar transistor 5 is connected to the anode of diode 6, the cathode thereof is connected to power supply 1. The drain voltage of depletion mode NMOS 4 is connected to an output terminal 7. The conventional temperature sensor circuit is configured as described above.
The conventional temperature sensor circuit 400 is formed of a first current supply circuit and a second current supply circuit. The first current supply circuit, the output current thereof exhibits a positive temperature dependence, is formed of Darlington transistor 90, formed of first bipolar transistor 3 and second bipolar transistor 5, and diode 6 (Zener diode that works as a temperature detecting section). The output current from the first current supply circuit is injected into depletion mode NMOS 4 that works as the second current supply circuit. Conventional temperature sensor circuit 400 is designed such that the voltage at the output terminal 7 exhibits a positive temperature dependence.
Specifically, the first and second current supply circuits constituting temperature sensor circuit 400 are formed on semiconductor substrate 500, on which a power switching device (power NMOS 60) that controls the electric power fed to load 70 is formed. As the temperature of semiconductor substrate 500 rises due to the operation of the power switching device, diode 6 that works as a temperature detecting section constituting the first current supply circuit is in a state in which a high leakage current flows through diode 6. In other words, Darlington transistor 90 is in a state in which the impedance thereof is small.
As power supply 1 is connected to temperature sensor circuit 400 in the state in which the impedance of Darlington transistor 90 is small, a gate current (leakage current) flows to the first bipolar transistor 3 in the front stage of Darlington transistor 90 constituting the first current supply circuit. The gate current is amplified by the second bipolar transistor 5 in Darlington transistor 90, making a high collector current flow. The high collector current flows into depletion mode NMOS 4 (depletion-type n-channel MOSFET working for a constant current circuit) constituting the second current supply circuit.
Since the gate of depletion mode NMOS 4 is fixed at the potential of ground 2, the current flowing into depletion mode NMOS 4 increases almost instantaneously to the value of the current that flows through depletion mode NMOS 4, the gate of which is fixed at the potential of ground 2. The increasing current enters a saturation region as described in FIG. 8, causing a constant current. FIG. 8 is a curve relating the drain voltage and the drain current of depletion mode NMOS 4 with each other, the gate voltage of which is fixed at the ground potential.
Output terminal 7 is connected to the gate of a MOSFET in a not-shown signal processing circuit. Any current does not flow through output terminal 7. In the state described above, the voltage at output terminal 7 is determined by the ratio of the impedance of depletion mode NMOS 4 and the impedance of Darlington transistor 90. When the temperature of semiconductor substrate 500 is low, the leakage current flowing through diode 6 is small. Therefore, since the gate current of first bipolar transistor 3 is small, the impedance of Darlington transistor 90 is larger than the impedance of depletion mode NMOS 4. Therefore, the voltage at output terminal 7, equal to the drain voltage of depletion mode NMOS 4, is low.
Since the leakage current flowing through diode 6 is high when the temperature of semiconductor substrate 500 is high, the gate current of first bipolar transistor 3 is high and the impedance of Darlington transistor 90 becomes low. Therefore, the voltage at output terminal 7, equal to the drain voltage of depletion mode NMOS 4, is high.
FIG. 9 is a wave chart describing the waveforms of the power supply voltage, semiconductor substrate temperature and output terminal voltage, when a power supply is connected to the conventional temperature sensor circuit. In FIG. 9, the solid lines represent the characteristics when the temperature of semiconductor substrate 500 is low and the dotted lines represent the characteristics when the temperature of semiconductor substrate 500 is high. FIG. 10 is an expanded wave chart describing the waveforms in the period B in FIG. 9. The symbols T◯, α, and T1 in FIG. 10 will be used in the descriptions of FIG. 3 later.
As power supply 1 is connected to conventional temperature sensor circuit 400 when the temperature of semiconductor substrate 500 is low, power NMOS 60 formed on semiconductor substrate 500 works, raising the temperature of semiconductor substrate 500 finally to a certain temperature. The certain temperature is the temperature of semiconductor substrate 500 when power NMOS 60 is conducting the steady state operations thereof. In the period, in which the temperature of semiconductor substrate 500 is rising, the output terminal voltage also rises. Any overshoot is not caused almost in the output terminal voltage at the instance, at which the power supply is connected.
When the temperature of semiconductor substrate 500 is high enough to destroy power NMOS 60 connected to load 70 (the line A in FIG. 9), the voltage at output terminal 7 rises as high as a predetermined voltage E. As the voltage at output terminal 7 reaches the predetermined voltage E, an interrupting signal is fed to power NMOS 60 to turn off power NMOS 60 and to prevent power NMOS 60 from being broken down.
Unexamined Laid Open Japanese Patent Application Publication No. Sho. 61 (1986)-233330 (Page 1, Right-hand-side column, Line 19—Page 2, Left-hand side column, Line 8; and FIG. 7) discloses a temperature sensor circuit that connects a constant current circuit to a Darlington transistor. As power supply 1 is connected in a state, in which the temperature of semiconductor substrate 500 is high as described by the dotted lines in FIG. 9, a high in-rush current flows instantaneously to depletion mode NMOS 4 through Darlington transistor 90. As a high in-rush current flows as described above, the drain voltage of depletion mode NMOS 4 exceeds the saturation voltage to the higher side instantaneously as shown in FIG. 8, causing an overshoot.
At the time point, at which the overshooting voltage reaches the predetermined voltage E, an interrupting signal is fed to power NMOS 60, turning off power NMOS 60. As power NMOS 60 is turned off, the electric power is not fed to load 70. As described above, FIG. 10 is an expanded wave chart describing the waveforms in the period B in FIG. 9. As described in FIG. 10, the voltage at output terminal 7 is overshooting greatly at the time of power supply connection. In describing the output terminal voltage in FIG. 10, a solid line represents the output terminal voltage when the temperature of semiconductor substrate 500 is high and a broken line the output terminal voltage when the temperature of semiconductor substrate 500 is low.
As described above, power NMOS 60 is turned off and the electric power feed to load 70 is interrupted at the time point, at which the overshooting voltage reaches the predetermined voltage E. In other words, electric power is not fed to load 70 even when power supply 1 is connected.
The above-described reference does not describe anything on preventing an overshooting voltage from causing at the output terminal at the instance of connecting a power supply. In addition, the above-described reference does not describe anything on the use of a depletion mode NMOS for the constant current circuit.
In view of the forgoing, it would be desirable to obviate the problems described above. It would be also desirable to provide a temperature sensor circuit that facilitates preventing a too-high overshooting voltage from causing at the output terminal thereof when a power supply is connected to the temperature sensor circuit.